Aluminum alloys are used for a number of resterilizable medical device applications, such as sterilization containers and trays, which can have large amounts of surface area. Containers and trays are used to retain the devices to be sterilized. Normally, a sterilization load comprises all the devices to be sterilized that are placed in packaging, which could be a container, tray, wrapping barrier, and/or pouch. Sterilization containers are designed to allow sterilization of medical devices and can be used to store the medical devices after sterilization and prior to use for a predetermined period. Containers typically comprise at least one filter that acts as a microbial barrier but allows sterilizing gas or vapor to diffuse into and out of the container. The instruments in the container can therefore remain sterile during storage because the container functions as a microbial barrier, preventing the instruments from becoming re-contaminated and nonsterile. On the other hand, an instrument sterilization tray normally does not comprise a gas or vapor permeable microbial impermeable barrier. The tray itself cannot maintain the sterility of the instrument in the tray after the sterilization process. It requires gas or vapor permeable and microorganism impermeable wrapping barrier outside of the tray to allow the diffusion of sterilizing agent into and out of the wrapped tray, to prevent the penetration of microorganisms into the wrapped tray, and to maintain the sterility of devices in the wrapped tray after sterilization. The tray may have a lid and the lid does not constitute a sterile barrier. Depending on the need and application, various dimensions of container and tray can be manufactured. An advantage of sterilization containers over trays for some users is that they eliminate the large amounts of disposable sterilization wrap used when sterilizing instruments in trays. This reduces disposal costs and environmental concerns.
Sterilization containers or trays commonly are made from anodized aluminum. Aluminum is used because of its light weight, good thermal conductivity, and corrosion resistance properties. Aluminum is usually anodized to improve its durability as well as resistance to corrosion (such as described in Mil Standard MIL-A-8625F for anodized aluminum, US Department of Defense, Military Specification MIL-A-8625F, pp. 1-19, Sep. 10, 1993). Standard anodized aluminum surface finishes contain very fine internal porosity, and a sterilizing agent such as hydrogen peroxide vapor can be absorbed into the porous layer. For a sterilization load with a large surface area, the absorption of sterilizing agent such as hydrogen peroxide may interfere with sterilization efficacy by reducing the amount of hydrogen peroxide available in the vapor phase in the chamber. This can be due to a combination of
1) large amounts of surface area of the device in combination with the large effective surface area of the anodization layer, and
2) penetration or diffusion of the hydrogen peroxide vapor past the outer liquid-repellent coating on the anodized surface to become absorbed by the inner porous oxide layer.
In the typical aluminum anodization process, such as a Type II process (U.S. Pat. No. 5,658,529 and U.S. Military Specification Mil-A-8625E, “Anodic Coatings for Aluminum and Aluminum Alloys,” Apr. 25, 1988, both of which are incorporated herein by reference in their entirety) aluminum is anodized in an aqueous solution of 95 mL/L H2SO4 for 30 minutes with a direct current of 15 to 21 volts and at a density of 9 to 12 A/sq. ft. Following the anodizing step, the aluminum oxide layer is sealed in boiling water or an aqueous solution of nickel acetate (5-5.8 g/L) for 30 minutes. The sealing process causes hydration of the aluminum oxide layer, which makes it impermeable to liquid.
Anodized aluminum coatings are inherently porous. This is due to the fact that the coating is produced electrolytically by anodic oxidation of aluminum. As the oxidized layer becomes thicker, there are two basic competing processes that occur (Wernick, et al, “The Surface Treatment and Finishing of Aluminum and its Alloys,” Vol. 1, 5th Ed., ASM International, Metals Park, Ohio, Chap. 6, p. 290). At the same time as the oxide coating is forming, it is being redissolved by the electrolyte. As the oxide layer builds up, it tends to limit the electrolytic current flow, because the oxide layer is relatively non-conducting. However, due to the re-dissolution of the oxide, pores begin to form in the oxide layer. These pores produce channels of high conductivity by locally thinning the oxide layer, allowing the liquid electrolyte to penetrate close to the metal substrate, while oxide and hydroxide crystals build up on the surface of the film, increasing the overall thickness. As long as the pores remain accessible to the electrolyte, the current can continue to flow and the film will grow further. The solubility of the oxides and hydroxides in the electrolyte can affect the porosity. Borate and tartrate electrolytes, with a low solubility for the oxides, tend to produce thin, dense coatings with low porosity. The coatings stop growing at relatively low thickness due to the high resistance of the less porous oxide layer. Sulfuric acid electrolytes tend to allow for faster dissolution of the oxide, allowing the pores to form and providing practical film thickness of 0.1 to 1.2 mm.
The most reliable non-destructive method for measuring film thickness uses eddy-current measurements (Wernick, et al., Chap. 12, p. 864). The eddy-current method, which is practical, non-destructive, fast and economical, is commonly used in commercial anodization facilities to verify film thickness and as a QC measurement technique. Research methods to measure film porosity have included lead acetate absorption, oil absorption, gas absorption (BET method), toluene absorption, electrolytic pore filling, reflectance methods, dielectric constant measurements, electron microscopy, and permeability (Wernick, et al, Chap. 12, pp. 878-882). However, porosity measurements, which tend to be time consuming and expensive, are not commonly used commercially as a process control or QC method.